Thermal transfer printing dyesheet

ABSTRACT

A thermal transfer dyesheet comprising an elongated substrate supporting print-size portions of first, second and third colour dyecoats arranged in a repeated sequence, said dyecoats each comprising one or more thermal transfer dyes dissolved or dispersed in a polymeric binder, characterised in that at least one of the second and third colour dyecoats has a clawback factor in respect of the first or second colour respectively, with a value in the range 1±0.3.

The invention relates to dyesheets for thermal transfer printing, andespecially to dyesheets capable of providing prints with good colourrendition.

Thermal transfer printing is a generic term for processes in which oneor more thermally transferable dyes are caused to transfer from adyesheet to a receiver in response to thermal stimuli. Using a dyesheetcomprising a thin substrate supporting a dyecoat containing one or moredyes uniformly spread over an entire printing area of the dyesheet,printing can be effected by heating selected discrete areas of thedyesheet while the dyecoat is pressed against a dye-receptive surface ofa receiver sheet, thereby causing dye to transfer to corresponding areasof the receiver. The shape of the pattern transferred is determined bythe number and locations of the discrete areas which are subjected toheating. Full colour prints can be produced by printing with differentcoloured dyecoats sequentially in like manner, and the differentcoloured dyecoats are usually provided as discrete uniform print-sizeareas in a repeated sequence along the same dyesheet.

High resolution photograph-like prints can be produced by thermaltransfer printing using appropriate printing equipment, such as aprogrammable thermal print head or laser printer, controlled byelectronic signals derived from a video, computer, electronic stillcamera, or similar signal generating apparatus. A typical thermal printhead has a row of tiny selectively energizable heaters, spaced to printsix or more pixels per millimeter, often with two heaters per pixel.Laser printers require absorbers to convert the laser radiation to heat,usually in or under the dyecoat, and similarly produce the print bytransferring dyes to the receiver pixel by pixel.

Like colour printing of various other technologies, thermal transferprinting is based on a subtractive three colour system, using yellow,magenta and cyan colours (not precluding the addition of black). Inaddition to their characteristic absorptions in the green, magenta dyestypically provide substantial blue absorption, and cyan dyes typicallyhave substantial absorption of green wavelengths in addition to theircharacteristic absorption of red light. Yellow dyes absorb in the blueto provide their characteristic colour, and the additional absorptionsat shorter wavelengths are generally outside the visible region, so donot contribute to the colour perceived.

Although such dyes may have good colour on their own, when they aremixed to produce other colours, these additional shorter wavelengthabsorptions can become a problem, in that they can distort the colourrendition obtained. Thus for example, yellow and magenta dyes arecombined to produce red, but a high blue absorption by the magenta willprovide a yellowness in addition to that of the yellow dye used, with aconsequence that the resulting red will be more orange than wouldotherwise be the case. In addition to a distorted colour balance, thecolour rendition can suffer from a dullness, where the colours appearless bright and the prints thereby less appealing.

This phenomenon is neither new nor confined to thermal transferprinting, and various correction techniques have previously beendeveloped in the various different colour reproducing technologies, suchas the masking used in conventional colour printing, for example. Inthermal transfer printing, colour correction can be obtained bymatrixing, a technique in which the control signals to the printer areadjusted for each pixel so as to correct the amount of each dyetransferred in a manner and to an extent which compensates for any othercorresponding absorption transferred in another dye to that same pixel,either before or subsequently. Thus, using the above example asillustration, the orangeness of the red may be corrected by transferringless of the yellow dye, where it is to be overprinted with the magentahaving high absorption in the blue, whereas elsewhere the full amount ofyellow is transferred by applying the yellow control signal unadjusted.

As will be appreciated, such corrections are complex, depending not onlyon whether there will be overprinting by either of the other dyes andwhether such dyes have relevant unwanted absorptions, but also on whatamounts of those other dyes are to be transferred to that pixel. Theadjustment for each colour needs to be recalculated and applied to thecontrol signal for each pixel, of which there are typically about 0.4 to1.5 million per A4 sheet, depending on the resolution of the printing.Moreover, the program controlling such corrections also needs to bechanged for different manufacturer's dyesheets because of theirdifferent chemical compositions.

We have now devised a dyesheet which makes such correctionsautomatically, and hence avoids any requirement to correct the controlsignals in the known manner described above. This is based on twodiscoveries. The first is that where one colour is overprinted byanother, clawback occurs in which some of the previously transferred dyeis removed from the receiver while the other dye is being transferred tothe same pixel. Thus to use the same illustration as before, whilemagenta dye is being transferred to one of the pixels of the print, someof the previously printed yellow dye is removed from that same pixel byclawback, and the degree of clawback is a property of the dyesheetcomposition. The second discovery was that the amount of clawback whichoccurs with a given dyesheet is proportional to the amount of the laterdye transferred, and not on the amount of the earlier dye for which theclawback is occurring. Thus again using the same example to illustratethis, we find that the more blue absorption capacity which is added to apixel as an unwanted side effect of the magenta dye, the greater is theamount of blue absorption lost by clawback of the yellow dye by the sameoperation. We have now been able to use these two observations to selectthe novel compositions defined below, which automatically compensate forthe presence of these unwanted absorptions by clawback.

Accordingly, the present invention provides a dyesheet comprising anelongated substrate supporting print-size portions of first, second andthird colour dyecoats arranged in a repeated sequence, said dyecoatseach comprising one or more thermal transfer dyes dissolved or dispersedin a polymeric binder, characterised in that at least one of the secondand third colour dyecoats has a clawback factor in respect of the firstor second colour respectively, with a value in the range 1±0.3.

Thus where the dyesheet has the colours arranged in the conventionalorder of yellow, magenta and cyan, that referred to above as "first"will be yellow, "second" will be magenta, and "third" will be cyan.

The clawback factor is calculated by printing the first colour to adensity of 1.4 then overprinting this with a gradient of the secondcolour. The optical density (OD) loss due to clawback is defined as thedifference between the sum of the densities of the two coloursseparately and the density of the overprint. The clawback factor is thegradient of the OD loss against the unwanted overprint density and isfound by performing a linear regression on the data. If the clawbackfactor were 1, then the effect of clawback would compensate for theunwanted density of the overprint dye. We have found that dyesheets withclawback factors in the range 1±0.3, for at least one, preferably both,of the pairs of adjacent colours, leads to prints with good colourrendition. The clawback factor of the first colour when printing thethird, normally the yellow-cyan clawback, is generally less significant,but we still prefer a first-third colours clawback factor similarly tofall within that specified range.

The nearer the value of 1 that the clawback factors become, the betterthe colour rendition, and we particularly prefer that at least one, butpreferably all, lie within the range 1 ±0.1.

EXAMPLES

The invention is illustrated by a comparison of a first dyesheet(Dyesheet A) prepared according to the invention and observed to giveprints of good colour rendition, and a second (Dyesheet B) which was aknown commercially available dyesheet that gave prints with a visiblecolour imbalance. Taking pairs of colours in turn, and printing one atvarying densities onto the other preprinted at a standard density,clawback factors were calculated from the measured optical densities inthe manner described above. A measurement of the unprinted receiver wasalso made, and subtracted from the print's optical densities toeliminate any off-white bias that may be contributed by the receiver.

The receiver used in these measurements had a substrate of 150 μmMelinex 990, ICI's white polyester film, and receiver coat of thefollowing composition:

    ______________________________________    Vylon 200          100 parts by weight    Cymel 303          1.4 parts by weight    Tegomer H--Si 2210 0.7 parts by weight    catalyst           0.4 parts by weight    Tinuvin 900        1.0 parts by weight    ______________________________________

wherein Vylon 200 is a dye-receptive linear polyester from Toyobo. Cymel303 is a hexamethoxymethylmelamine oligomer sold by American Cyanamid.Tegomer HSi 2210 is a bis-hydroxyalkyl polydimethylsiloxane sold byGoldshmidt and is cross-linkable by the Cymel 303 under acid conditions.The catalyst used was a blocked p-toluene sulphonic acid, and Tinuvin900 is a UV stabiliser. The results are shown in the tables below.

    ______________________________________    Dyesheet A    Yellow-Magenta Clawback    Macbeth TR1224 Densitometer - Blue Filter Readings                 Adjusted For White    Yellow          Magenta  Red     Yellow                                 Magenta                                        Red   OD Loss    ______________________________________    1.42  0.12     1.32    1.33  0.03   1.23  0.13    1.41  0.17     1.38    1.32  0.08   1.29  0.11    1.45  0.23     1.4     1.36  0.14   1.31  0.19    1.46  0.34     1.34    1.37  0.25   1.25  0.37    1.45  0.43     1.36    1.36  0.34   1.27  0.43    ______________________________________     White  0.09     YellowMagenta Clawback Factor = 1.11

    ______________________________________    Yellow-Cyan Clawback    Macbeth TR1224 Densitometer - Blue Filter Readings                 Adjusted For White    Yellow Cyan   Green    Yellow                                 Cyan  Green OD Loss    ______________________________________    1.35   0.12   1.3      1.26  0.03  1.21  0.08    1.37   0.17   1.31     1.28  0.08  1.22  0.14    1.38   0.24   1.3      1.29  0.15  1.21  0.23    1.42   0.31   1.28     1.33  0.22  1.19  0.36    1.42   0.4    1.3      1.33  0.31  1.21  0.43    1.45   0.55   1.45     1.36  0.46  1.36  0.46    ______________________________________     White  0.09     YellowCyan Clawback Factor = 0.94

    ______________________________________    Magenta-Cyan Clawback    Macbeth TR1224 Densitometer - Green Filter Readings                 Adjusted For White    Magenta           Cyan    Blue    Magenta                                  Cyan  Blue  OD Loss    ______________________________________    1.33   0.17    1.34    1.25   0.09  1.26  0.08    1.38   0.24    1.38    1.3    0.16  1.3   0.16    1.39   0.36    1.4     1.31   0.28  1.32  0.27    1.39   0.49    1.44    1.31   0.41  1.36  0.36    1.42   0.64    1.52    1.34   0.56  1.44  0.46    1.42   0.92    1.56    1.34   0.84  1.48  0.7    ______________________________________     White  0.08     MagentaCyan Clawback Factor = 0.80

    ______________________________________    Dyesheet B    Yellow-Magenta Clawback    Macbeth TR1224 Densitometer - Blue Filter Readings                 Adjusted For White    Yellow          Magenta  Red     Yellow                                 Magenta                                        Red   OD Loss    ______________________________________    1.34  0.12     1.39    1.26  0.04   1.31  -0.01    1.37  0.22     1.41    1.29  0.14   1.33  0.1    1.4   0.35     1.45    1.32  0.27   1.37  0.22    1.43  0.49     1.5     1.35  0.41   1.42  0.34    1.44  0.62     1.59    1.36  0.54   1.51  0.39    1.41  0.76     1.71    1.33  0.68   1.63  0.38    ______________________________________     White  0.08     YellowMagenta Clawback Factor = 0.64

    ______________________________________    Yellow-Cyan Clawback    Macbeth TR1224 Densitometer - Blue Filter Readings                 Adjusted For White    Yellow Cyan   Green    Yellow                                 Cyan  Green OD Loss    ______________________________________    1.4    0.1    1.29     1.32  0.02  1.21  0.13    1.36   0.17   1.31     1.28  0.09  1.23  0.14    1.37   0.25   1.33     1.29  0.17  1.25  0.21    1.38   0.35   1.37     1.3   0.27  1.29  0.28    1.38   0.47   1.44     1.3   0.39  1.36  0.33    1.39   0.6    1.54     1.31  0.52  1.46  0.37    ______________________________________     White  0.08     YellowCyan Clawback Factor = 0.52

    ______________________________________    Magenta-Cyan Clawback    Macbeth TR1224 Densitometer - Green Filter Readings                 Adjusted For White    Magenta           Cyan    Blue    Magenta                                  Cyan  Blue  OD Loss    ______________________________________    1.33   0.15    1.34    1.25   0.07  1.26  0.06    1.38   0.23    1.38    1.3    0.15  1.3   0.15    1.39   0.35    1.4     1.31   0.27  1.32  0.26    1.39   0.48    1.44    1.31   0.4   1.36  0.35    1.42   0.64    1.54    1.34   0.56  1.46  0.44    1.42   0.93    1.89    1.34   0.85  1.81  0.38    ______________________________________     White  0.08     MagentaCyan Clawback Factor = 0.43

According to these results, Dyesheet A is a dyesheet according to thepresent invention, whereas Dyesheet B lies outside it.

The two dyesheets were also used to make full colour prints from thesame control signals. No corrections were applied to these signals tocompensate for differences in the dyes. The prints obtained fromdyesheet A were noticeably brighter, appearing to have colours whichwere truer to the original scene, than those made using Dyesheet B. Thissubjective comparison showed the prints made with dyesheets according tothe present invention, had a visibly improved colour rendition whencompared with the prints we made in similar manner from Dyesheet B, acommercial dyesheet presently available on the market.

I claim:
 1. A thermal transfer dyesheet comprising an elongatedsubstrate supporting print-size portions of first, second and thirdcolour dyecoats arranged in a repeated sequence, said dyecoats eachcomprising one or more thermal transfer dyes dissolved or dispersed in apolymeric binder, characterised in that at least one of the second andthird colour dyecoats has a clawback factor in respect of the first orsecond colour respectively, with a value in the range 1±0.3; whereinclawback occurs when some of a dye previously transferred to a receiveris removed while being overprinted by a subsequent colour, and theclawback factor is the ratio of the optical density loss throughclawback and the density of the subsequent colour at the samewavelength; the optical density loss being defined as the sum of thedensities of the two colours separately minus the density of theoverprint, at the wavelength characteristic of the previouslytransferred dye.
 2. A dyesheet as claimed in claim 1, characterised inthat both the second and third colour dyecoats have a clawback factor inrespect of the first or second colour respectively, with a value in therange 1±0.3.
 3. A dyesheet as claimed in claim 1, characterised in thatboth the second and third colour dyecoats have a clawback factor inrespect of the first colour with a value in the range 1±0.3.
 4. Adyesheet as claimed in any one of claims 1-3 characterised in that atleast one of the three clawback factors has a value in the range 1±0.1.